Compressed propane is often used as a convenient heating and cooking fuel. It can be transported in refillable tanks, or delivered by truck to stationary tanks. In many instances, the necessary valves and couplings pose a potential leakage hazard. If leakage is sufficient, propane gas, which is heavier than air, can accumulate and mix with the air, forming an explosive mixture. A propane tank must not be filled above capacity, normally 80% of volume, in order to accommodate thermal expansion of the gas and liquid in the tank. Approved tanks each include a pressure relief valve, which allows escape of propane in the event of overfilling, avoiding the risk of rupture of the tank with release of all of the propane. However, these relief valves themselves release propane into the atmosphere, which may reach dangerous levels. The risk of overfilling is increased in the case of transportable propane tanks, e.g., D.O.T. approved 20 lb. tanks. There therefore remains a need for a system to detect propane leakage around propane tanks, to provide an alert of the potential hazard.
A large number of explosive gas detectors and propane detectors are known. Available types, fall into four general categories:
1. calorimetric analysis of sample gas PA1 2. photometric analysis of sample gas PA1 3. semiconductor detectors PA1 4. laboratory, chemical and other detectors
A calorimetric detector seeks to determine the functional presence of combustible gasses by oxidizing any such gases present, and measuring the "excess heat" generated. These detectors, for example, employ a heated catalytic bead and non-catalytic head disposed on legs of a platinum resistance thermometer, measuring the differential bridge voltage due to the flammable gas induced heating of the catalytic bead with respect to the non-catalytic bead. New technologies allow micromachined devices to serve as combustible gas detectors, for example so-called microcanteliever devices developed by Oak Ridge National Labs.
A photometric gas detector seeks to detect the presence of gas by a signature pattern, for example the spectrographic characteristics of the gas. Typically, the characteristic absorption peaks of hydrocarbon gasses fall in the infrared range. The photometric characteristics of a gas may be measured locally, in a detector space, or remotely, such as by remote laser sensing techniques.
A semiconductor detector operates by selectively or semiselectively absorbing the gas to be measured to a semiconducting material, and measuring an alteration in the conductive properties of the semiconducting material. Typical semiconductors include tin oxide, lathough a number of other semiconducting materials may also be used. Semiconducting detectors typically have low selectivity, and respond to a broad range of gasses, for example lower hydrocarbons and alcohols, carbon monoxide, hydrogen sulfide, and other gasses.
Lastly, there are a number of specialty detectors, for example those used in laboratory analyses, color change detectors, mass spectroscopy, and the like, which may also detect gasses.
In a domestic environment, propane gas is used to provide heat, for cooking, and for barbecues. While some types of detectors are available which will detect leaking propane, their use is limited, especially in uncontrolled environments. Thus, the available domestic flammable gas detectors are intended for permanent mounting, continuous operation and line current operation.
Propane tanks for domestic barbecues are typically refilled and transported. U.S. Department of Transportation (D.O.T.) regulations carefully describe the construction and features of these tanks, in order to ensure safe transport. These tanks are each provided with a safety relief valve, near the main valve, which vents in the event of overpressure. Propane tanks for barbecues are approximately 18" high and 12" diameter, with a valve system centered on the top surrounded by a handle/protective cage about 270.degree.. The valve handle extends upward, below the upper extent of the handle/protective cage, and has a nozzle which projects at right angles toward the open portion of the handle/protective cage. The handle/protective cage has three openings, a larger of which serves as a handle. The base of the tank includes a conic section between a spherical lower portion of the tank and the ground.
Typically, the sensing system used in distributed combustible gas detectors includes an electronic sensor detecting the presence of combustible gas, such as a heated sensor with a flame arrestor, forming an "intrinsically safe" module (i.e., low probability that the detector may itself ignite the propane). An alternate type of sensing system employs a semiconductor which responds to the presence of various Lyases, such as propane. For example, Motorola Senseon MGS-1200 sensor, Figaro TGS2612, Figaro TGS 813, English Electric Valve (EEV) heated catalytic bead combustible gas sensor, or similar types may be used.
Carbon monoxide is a normal product of propane combustion, and in closed environments, this may also become a hazard. Known carbon monoxide sensors include the Figaro TGS2440. Figaro TGS800 and Motorola Senseon MGS-1100. The later two sensors have marked cross sensitivity with propane, and thus may be used to detect both hazards, but cannot alone distinguish them. Carbon monoxide is typically sensed with semiconductor-type sensors or electrochemical detectors.
As a result of the permissible characteristics, including allowable power consumption, warm-up time and other constraints, installed domestic sensors can easily employ sensors with moderate or high current consumption, long warm-up stabilization time and bulky size. Known portable personal safety devices also employ these types of sensors: however the systems provide a limited battery life and may be relatively heavy due to the batteries necessary to run the integral sensor heaters.
Heated sensors, such as the Senseon series of gas detectors, are micromachined and employ an element covered with a catalytic material, which is electrically heated. For example, a Senseon MGS 1200 type device employs a tin oxide film. The temperature of the sensor is measured, for example as compared to a like sensor element absent the catalyst.
These semiconductor sensors may also be used to detect combustible gasses. In the MGS 1200, propane interacts with the heated catalyst to alter its conductivity. In other systems, the degree of temperature rise of the catalytic element is related to the amount (caloric content) of combustible gas present.
Catalytic and semiconductor sensors may require long stabilization times, in order to eliminate non-specific absorption to the catalyst or other interfering influences.
Thus, The following U.S. Patents, as well as references cited therein, incorporate herein by reference, relate to gas detection and associated technologies: U.S. Pat. Nos. 5,879,631; 5,828,307; 5,807,098; 5,797,358; 5,709,222; 5,650,024; 5,379,026; 4,709,150; 4.437,005; 4,323,777; 5,721,430; 3,662,171; 5,444,249; 5,608,219; 5,070,244; 5,464,983; 4,916,437; 4,694,174; 4,560,875; 5,475,222; 3,861,809; 5,026,992; 4,647,777; 4,567,366; 4,958,076; 4,500,207; 4,899,053; 4,489,239; 4,871,916; 4,853,543; 5,637,872; 5,372,785: 5,250,170; 4,580,439; 5,012,671; 5,003,812; 4,164,539; 5,218,347; and 4,614,669.
U.S. Pat. No. 5,813,394, expressly incorporated herein by reference, relates to a cooking grill with a flame detector, for relighting the flame if blown out and providing automatic shutoff.